GL primitives can have either flat or smooth
shading. Smooth shading, the default, causes the computed colors of vertices
to be interpolated as the primitive is rasterized, typically assigning
different colors to each resulting pixel fragment. Flat shading selects
the computed color of just one vertex and assigns it to all the pixel fragments
generated by rasterizing a single primitive. In either case, the computed
color of a vertex is the result of lighting if lighting is enabled, or
it is the current color at the time the vertex was specified if lighting
is disabled.

Flat and smooth shading are indistinguishable for points. Starting
when glBegin is issued and counting vertices and primitives from 1, the
GL gives each flat-shaded line segment $i$ the computed color of vertex
$i ~+~ 1$, its second vertex. Counting similarly from 1, the GL gives each
flat-shaded polygon the computed color of the vertex listed in the following
table. This is the last vertex to specify the polygon in all cases except
single polygons, where the first vertex specifies the flat-shaded color.

Primitive Type of Polygon $i$ Vertex

Single polygon ($ i ~==~ 1 $) 1

Triangle
strip $i ~+~ 2$

Triangle fan $i ~+~ 2$

Independent triangle $ 3 i$

Quad strip $2
i ~+~ 2$

Independent quad $ 4 i $

Flat and smooth shading are specified
by glShadeModel with mode set to GL_FLAT and GL_SMOOTH, respectively.